The present invention is directed to a thermally activated (powered by heat) heat pump. The heat pump is of an integrated type with a single cycle and a single working fluid which flows undivided in series through a thermal engine driving portion and then through a heat pump portion.
There are many terrestrial and space applications where replacement of electrically activated heat pumps by thermally activated heat pumps would result in major savings in energy and cost. An example of such a use is air conditioning units for houses and buildings. If efficient thermally activated heat pumps can be developed, then most of such air conditioning units could be powered by natural gas with large savings. The use of thermally activated heat pumps is of sufficient importance that the United States Department of Energy supports a significant research and development program on such devices. However, all presently supported projects employ separate heat pumping and mechanical power supplying units, for example, internal combustion engines based on the Stirling, Brayton or Rankine cycles driving heat pumping units.
When heat pumps are used for heating purposes, those that are thermally activated could provide significantly more heat than the heating value of fuel used to power the heat pump. This potentially could be achieved without expenditure of electrical energy. This means, for example, if gas or other fuel is used to heat a building, by utilizing a thermally activated heat pump substantially more heat (heating value) could be provided to the building. The main thermodynamic reason behind this is that fuel can burn at considerably higher temperature than needed for heating. Insertion of a thermally activated heat pump decreases thermodynamic irreversibilities and improves utilization of fuel several fold.
U.S. Pat. No. 3,621,667 to Castillo has proposed a slight modification over the usual thermally activated heat pump in which a thermal engine, a Rankine cycle vapor turbine, drives a vapor compressor cycle heat pump. In the Castillo patent, the only innovation is that the vapor exiting the turbine cycle and the vapor exiting the compressor cycle are then combined and condensed in a single common condenser. The system suffers from the usual problems of a Rankine cycle. These problems are the need to superheat the vapor before it enters the vapor turbine, inability of the vapor turbine to handle moist vapor, existence of pinch points and poor matching of heat exchange curves of a heat source fluid and of the vapor, resulting in lower thermal efficiency.
U.S. Pat. No. 4,438,638 to Hays et al discloses the modification of a conventional electrically or mechanically driven heat pump. A throttling pressure let-down expander for a condensate (liquid) is replaced by DeLaval stationary two-phase nozzles. In the nozzles, saturated condensate flashes into low quality two-phase flow. In this way, most of the enthalpy drop in the pressure let-down expansion is converted into kinetic energy of two-phase flow (a major part of it being in the liquid phase). A good part of the kinetic energy of the liquid is converted into useful mechanical energy in reaction hydraulic turbine rotor. That is, only the stationary nozzle experiences two-phase flow while only the liquid passes through the turbine rotor. In practice, this stationary two-phase nozzle/hydraulic turbine rotor combination has proven to have a turbine efficiency of up to only 43%. Energy savings predicted by the inventors is only about 5%. This is due to the fact that low available enthalpy drop is usually encountered when flashing saturated liquid between two low, heat absorption and heat rejection pressures. Since the efficiency of the stationary two-phase nozzle/hydraulic turbine combination turned out to be lower than predicted by Hays et al., actual energy savings with this system is lower than 5%.
U.S. Pat. No. 1,275,504 to Vuilleumier discloses a thermally activated integrated heat pump which is supposed to use a single fluid flowing as a single stream consecutively through a thermal engine driving cycle and a heat pumping cycle. There has been considerable research and development on this system during the last 15 years. No continuous flow (steady state) embodiment using this cycle has been achieved to date. The system is quite complicated with two reciprocating pistons connected by an involved mechanism and four recuperative and two regenerative heat exchangers in which flow is injected intermittently. In practice, the efficiency of these systems has not appreciably approached its ideal lossless theoretical value. Nevertheless, the concept is still promising as recent activity indicates.
U.S. Pat. No. 3,621,667 to Mokadam discloses another thermally activated continuous flow integrated heat pump concept. The concept is shown schematically on FIG. 1 with its thermodynamic P-v and T-s diagrams given in FIGS. 2 and 3. In this cycle, a thermodynamic working fluid is first heated as a high pressure liquid to its saturation temperature in heater 1 by addition of high temperature heat Q.sub.in f. The working fluid is then flashed through a stationary two-phase flow DeLaval (converging-diverging) nozzle 2 achieving the lowest temperature D. Subsequently, the working fluid is separated in a separator 3, with most of the liquid in the two-phase stream being evaporated by addition of low temperature heat Q.sub.in e in an evaporator 4. Next, the two-phase flow is decelerated in an expanding diffuser 5 which converts kinetic energy of the fluid into an increased pressure (and increased temperature) state F. Condensation is accomplished in a condenser 6 by a rejection of heat Q.sub.out c. The condensed liquid is pumped to a higher pressure by a pump 7. Subcooled liquid then enters the heater 1 completing the cycle. It is understood that a prototype of this device has never been built. If this system could be made to work as successfully as the first order thermodynamic analysis indicates, it certainly would be a very useful device with many applications. It would be considerably simpler and more reliable than other thermally activated heat pumps presently being developed. In addition, it would be more efficient. However, more detailed fluid dynamic analysis indicates that there would be problems with designing and operating an efficient two-phase diffuser (process E-F on FIGS. 1, 2, 3) which will cause a failure of the whole cycle. It is known that two-phase flow diffusers are inherently inefficient in practice. Namely, most or an appreciable part of the kinetic energy at the entrance to a diffuser is carried by the liquid phase. In practice, this liquid phase does not get appreciably slowed down while passing through a diffuser. In this way, most of the kinetic energy of the liquid is not recovered but is uselessly dissipated. If the liquid is separated out upstream of the diffuser, as indicated in FIG. 1, then the diffuser will be equally dissipated through friction with the wall. Since the pressure and enthalpy drops across the stationery two-phase nozzle are high, achieved velocities would also be very high. The high velocities would cause high losses in the nozzle as well.